1. Field of the Invention
The present invention relates to an air fuel ratio control system for internal combustion engines which utilizes an oxygen sensor to detect an air fuel ratio, and which performs feedback control based on the results of the detection.
2. Discussion of Background
An internal combustion engine which is mounted on an automobile has had a catalyst in an exhaust pipe in order to purify exhaust gas. It has widely come in practice that an oxygen sensor is utilized to detect exhaust gas composition, and that a feedback control is carried out to cause the actual air fuel ratio to become a theoretical air fuel ratio based on the results of the detection, thereby ensuring good purification efficiency.
Referring to FIG. 3, there is shown a fuel-injection system wherein such air fuel ratio control is made. In FIG. 3, reference numeral 1 designates an internal combustion engine having four cylinders. Reference numeral 2 designates an intake pipe which supplies intake air to the internal combustion engine 1. Reference numerals 3a, 3b, 3c and 3d designate injectors which are arranged in an intake manifold of the intake pipe 2 so as to correspond to the respective cylinders of the internal combustion engine 1, and which supplies fuel to the respective cylinders. Reference numeral 4 designates a throttle valve which is arranged in the intake pipe 2 to adjust the quantity of the intake air to be supplied to the engine. Reference numeral 5 designates an air flow sensor (AFS) which detects the quantity of the intake air which will be introduced into the engine. Reference numeral 6 designates an exhaust pipe which is used to discharge exhaust gas from the engine 1. Reference numeral 7 designates an oxygen sensor which is arranged in the exhaust pipe 6 to detect the concentration of oxygen in the exhaust gas. Reference numeral 8 designates a catalytic converter which is arranged in the exhaust pipe 6 to purify the exhaust gas by use of a catalyst. Reference numeral 9 designates a revolution sensor which generates a pulse signal depending on the revolution of the engine 1. Reference numeral 10 designates an electronic control unit (ECU) which computes a required amount of fuel supply based on input information indicative of engine operating conditions which are detected by the air flow sensor 5, the oxygen sensor 7, the revolution sensor 9 and the like. The electronic control unit produces a signal indicative of the computed results to drive the injectors 3a-3d.
In such arrangement, the electronic control unit 10 uses as the main information an intake air quantity signal which is detected by the air flow sensor 5, and a pulse signal which is obtained by the revolution sensor 9. The electronic control unit 10 computes a basic fuel-injection quantity based on the main information to determine a driving pulse width for the injectors 3a-3d. The injectors 3a-3d are pulse-driven in sequence based on a given revolution signal, and the fuel is injected into an intake manifold in the amount corresponding to the pulse width.
In the control wherein such computation is carried out to set a required quantity of the fuel, the actual air fuel ratio does not always correspond to the theoretical air fuel ratio. The actual air fuel ratio varies in the range of about 4 to about 5% with respect to the theoretical air fuel ratio due to an operation error of the injectors 3a-3d and a detection error of the air flow sensor 5. In order to cope with this problem, the conventional fuel-injection system includes the oxygen sensor 7 to detect the concentration of oxygen in the exhaust gas, and carries out feedback control based on an output signal from the oxygen sensor, thereby trying to cause the actual air fuel ratio to correspond to the theoretical air fuel ratio. The oxygen sensor 7 outputs an output voltage, the sign of which reverses using the theoretical air fuel ratio as a reference as shown in FIG. 4. The quantity of fuel is increased and decreased depending on the reversal of the output voltage to carry out feedback control, having the theoretical air fuel ratio as the target as shown in FIG. 5. In that manner, the catalytic converter 8 can ensure the purification efficiency of the exhaust gas at the maximum, thereby minimizing effect on the environment.
In such a conventional air fuel ratio control system, the output signal from the oxygen sensor 7 in fact indicates an average of the values which could be gotten in the plural cylinders. As a result, there is a variation in the actual air fuel ratio of the respective cylinders with respect to the theoretical air fuel ratio as shown in FIG. 6. It means that the purification efficiency in the catalytic converter 8 can not be maintained an ideal value, causing the exhaust gas from the internal combustion engine 1 to have adverse effect on the environment.